Re: [PATCHv2 REBASE] KVM: prevent spurious exit to userspace during task switch emulation.

[Gleb Natapov <gleb@redhat.com>]

If kvm_task_switch() fails code exits to userspace without specifying
exit reason, so the previous exit reason is reused by userspace. Fix
this by specifying exit reason correctly.

---
Changelog:
 v1->v2:
  - report emulation error to userspace instead of ignoring it silently.

Signed-off-by: Gleb Natapov<gleb@redhat.com>
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 4cb3aa6..6238eea 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -2265,8 +2265,14 @@ static int task_switch_interception(struct vcpu_svm *svm)
 	     (int_vec == OF_VECTOR || int_vec == BP_VECTOR)))
 		skip_emulated_instruction(&svm->vcpu);
 
-	return kvm_task_switch(&svm->vcpu, tss_selector, reason,
-			       has_error_code, error_code);
+	if (kvm_task_switch(&svm->vcpu, tss_selector, reason,
+				has_error_code, error_code) == EMULATE_FAIL) {
+		svm->vcpu.run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
+		svm->vcpu.run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
+		svm->vcpu.run->internal.ndata = 0;
+		return 0;
+	}
+	return 1;
 }
 
 static int cpuid_interception(struct vcpu_svm *svm)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 29a63d4..7e2f8d5 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3315,9 +3315,13 @@ static int handle_task_switch(struct kvm_vcpu *vcpu)
 		       type != INTR_TYPE_NMI_INTR))
 		skip_emulated_instruction(vcpu);
 
-	if (!kvm_task_switch(vcpu, tss_selector, reason, has_error_code,
-			     error_code))
+	if (kvm_task_switch(vcpu, tss_selector, reason,
+				has_error_code, error_code) == EMULATE_FAIL) {
+		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
+		vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
+		vcpu->run->internal.ndata = 0;
 		return 0;
+	}
 
 	/* clear all local breakpoint enable flags */
 	vmcs_writel(GUEST_DR7, vmcs_readl(GUEST_DR7) & ~55);
--
			Gleb.